Abnormal splicing of the leptin receptor in diabetic mice

Leptin impairs metabolic actions of insulin in isolated rat adipocytes

Leptin is an adipocyte hormone involved in the regulation of energy homeostasis. Generally accepted biological effects of leptin are inhibition of food intake and stimulation of metabolic rate in ob/ob mice that are defective in the leptin gene. In contrast to these centrally mediated effects of leptin, we are reporting here on leptin effects on isolated rat adipocytes. Leptin impairs several metabolic actions of insulin, i.e. stimulation of glucose transport, glycogen synthase, lipogenesis, inhibition of isoproterenol-induced lipolysis, and protein kinase A activation, as well as stimulation of protein synthesis. Insulin effects were reduced by leptin (2 nM) with a half-life of about 8 h. At low leptin concentrations (<1 nM), the insulin sensitivity was reduced leading to a shift to the right in the dose-response curve. At higher concentrations the responsiveness was diminished, resulting in nearly complete inhibition of insulin effects at >30 nM leptin. The IC50 value of leptin was 3.1 +/- 1 nM after 15 h of preincubation of adipocytes in primary culture. The natural splice variant des-Gln49-leptin exhibited a significantly lower potency. Adipocytes regained full insulin sensitivity within a few hours after leptin removal. The stimulation of glucose transport by vanadate was not affected by leptin. These data show specific and potent impairment of insulin action by leptin in the physiological concentration range of both leptin and insulin, which may be related to the pathophysiology of insulin resistance in both non-insulin-dependent diabetes mellitus and obesity.

Amino acid variants in the human leptin receptor: lack of association to juvenile onset obesity

The recently described putative lipostat system mediated in part by leptin and its hypothalamic receptor provides logical candidate genes for the molecular basis of inherited obesity in humans on the basis of the occurrence of profound obesity observed in obese and diabetic mice, in which the genes for leptin or its receptor, respectively, are mutated. In this study we tested the hypothesis that juvenile onset obesity in humans may be caused by leptin resistance mediated through genetic variations in isoforms of the hypothalamic leptin receptor. One hundred and fifty-six obese Danish men with a history of juvenile onset obesity were selected at the draft board examination with a body mass index (BMI) > or = 31 kg/m2. From the same study population a control group of 205 control subjects (mean BMI = 21,5 kg/m2) were randomly selected. Single strand conformational polymorphism scanning of genomic DNA from 56 obese subjects revealed a total of four amino acid variants located in coding exons 2, (Lys109Arg), 4 (Lys204Arg and Gln223Arg), and 12 (Lys656Asn), respectively. The codons 109, 223, and 656 variants were common, but their prevalence was not significantly different between obese and lean carriers with regard to allele or carrier frequency (p > 0.1 in each case). The codon 204 mutation was only found in one obese subject. In conclusion, it is unlikely that mutations in the coding region of the long isoform of the leptin receptor are a common cause of juvenile onset obesity.

Obesity, diabetes and functions for proopiomelanocortin-derived peptides

Melanocortin peptides (adrenocorticotropin (ACTH), alpha-,beta-, and gamma-melanocyte stimulating hormone (MSH), and fragments thereof) derived from proopiomelanocortin (POMC) have a diverse array of biological activities, many of which have yet to be fully elucidated. The recent cloning of a family of five distinct melanocortin receptors through which these peptides act has provided the tools to further our understanding of melanocortin peptide functions. Early work on melanocortin peptides focused on their roles in pigmentation, adrenocortical function, the immune, central and peripheral nervous systems. Although melanocortin peptides have long been known to affect lipolysis, characterisation of the melanocortin receptors has opened up several lines of evidence for important roles in the development of obesity, insulin resistance and type II diabetes. We present here a review of the current evidence for melanocortin peptides playing such a role, and based on this evidence, a model for melanocortin peptides and their receptors in maintaining energy balance.

Pediatric obesity. An overview of etiology and treatment

Pediatric obesity is a chronic and growing problem for which new ideas about the biologic basis of obesity offer hope for effective solutions. Prevalence of pediatric and adult obesity is increasing despite a bewildering array of treatment programs and severe psychosocial and economic costs. The definition of obesity as an increase in fat mass, not just an increase in body weight, has profound influence on the understanding and treatment of obesity. In principle, body weight is determined by a balance between energy expenditure and energy intake, but this observation does not by itself explain obesity. There is surprisingly little evidence that the obese overeat and only some evidence that the obese are more sedentary. Understanding of the biologic basis of obesity has grown rapidly in the last few years, especially with the identification of a novel endocrine pathway involving the adipose tissue secreted hormone leptin and the leptin receptor that is expressed in the hypothalamus. Plasma leptin levels are strongly correlated with body fat mass and are regulated by feeding and fasting, insulin, glucocorticoids, and other factors, consistent with the hypothesis that leptin is involved in body weight regulation and may even be a satiety factor (Fig. 2, Table 1). Leptin injections have been shown to reduce body weight of primates, although human clinical trials will not be reported until summer 1997. So many peptides influencing feeding have been described that one or more may have therapeutic potential (Fig. 2, Table 1). Although the complexity of pathways regulating body weight homeostasis slowed the pace of understanding underlying mechanisms, these complexities now offer many possibilities for novel therapeutic interventions (Fig. 2). Obesity is a major risk factor for insulin resistance and diabetes, hypertension, cancer, gallbladder disease, and atherosclerosis. In particular, adults who were obese as children have increased mortality independent of adult weight. Thus, prevention programs for children and adolescents will have long-term benefits. Treatment programs focus on modification of energy intake and expenditure through decreased calorie intake and exercise programs. Behavior-modification programs have been developed to increase effectiveness of these intake and exercise programs. These programs can produce short-term weight loss. Long-term losses are more modest but achieved more successfully in children than in adults. Several drug therapies for obesity treatment recently have been approved for adults that produce sustained 5% to 10% weight losses but experience with their use in children is limited. Identification of the biochemical pathways causing obesity by genetic approaches could provide the theoretic foundation for novel, safe, and effective obesity treatments. The cloning of leptin in 1994 has already led to testing the efficacy of leptin in clinical trials that are now underway. Although novel treatments of obesity are being developed as a result of the new biology of obesity, prevention of obesity remains an important goal.

In vivo effects of leptin-related synthetic peptides on body weight and food intake in female ob/ob mice: localization of leptin activity to domains between amino acid residues 106-140

In C57BL/6J ob/ob mice, a single base mutation of the ob gene in codon 105 results in the replacement of arginine by a premature stop codon and production of a truncated inactive form of leptin. These observations suggest that leptin activity may be localized, at least in part, to domains distal to amino acid residue 104. To investigate this possibility, we synthesized six overlapping peptide amides corresponding to residues 106-167 of leptin, and examined their effects on body weight and food intake in female C57BL/6J ob/ob mice. When compared with vehicle-injected control mice, weight gain by mice receiving 28 daily 1-mg i.p. injections of LEP-(106-120), LEP-(116-130), or LEP-(126-140) was significantly (P < 0.01) reduced with no apparent toxicity. Weight gain by mice receiving LEP-(136-150), LEP-(146-160), or LEP-(156-167) was not significantly different from that of vehicle-injected control mice. The effects of LEP-(106-120), LEP-(116-130), or LEP-(126-140) were most pronounced during the first week of peptide treatment. Within 7 days, mice receiving these peptides lost 12.3%, 13.8%, and 9.8%, respectively, of their initial body weights. After 28 days, mice given vehicle alone, LEP-(136-150), LEP-(146-160), or LEP-(156-167) were 14.7%, 20.3%, 25.0%, and 24.8% heavier, respectively, than they were at the beginning of the study. Mice given LEP-(106-120) or LEP-(126-140) were only 1.8% and 4.2% heavier, respectively, whereas mice given LEP-(116-130) were 3.4% lighter. Food intake by mice receiving LEP-(106-120), LEP-(116-130), or LEP-(126-140), but not by mice receiving LEP-(136-150), LEP-(146-160), or LEP-(156-167), was reduced by 15%. The results of this study indicate 1) that leptin activity is localized, at least in part, in domains between residues 106-140; 2) that leptin-related peptides have in vivo effects similar to those of native leptin; and 3) offer hope for development of peptide analogs of leptin having potential application in human or veterinary medicine.

The metabolic significance of leptin in humans: gender-based differences in relationship to adiposity, insulin sensitivity, and energy expenditure

Leptin is an adipocyte-derived hormone that interacts with a putative receptor(s) in the hypothalamus to regulate body weight. The relationship of leptin to metabolic abnormalities associated with obesity together with hormonal and substrate regulation of leptin have not been extensively studied. Therefore, 116 subjects (62 men and 54 women) with a wide range of body weight [body mass index (BMI), 17-54 kg/m2] were characterized on a metabolic ward with regard to body composition, glucose intolerance, insulin sensitivity, energy expenditure, substrate utilization, and blood pressure. Eighty-five of the subjects had normal glucose tolerance (50 men and 35 women), and 31 had noninsulin-dependent diabetes mellitus (12 men and 19 women). In both men and women, fasting leptin levels were highly correlated with BMI (r = 0.87 and r = 0.88, respectively) and percent body fat (r = 0.82 and r = 0.88, respectively; all P < 0.0001). However, men exhibited lower leptin levels at any given measure of obesity. Compared with those in men, leptin levels rose 3.4-fold more rapidly as a function of BMI in women [leptin = 1.815 (BMI)-31.103 in women; leptin = 0.534 (BMI)-8.437 in men] and 3.2 times more rapidly as a function of body fat [leptin = 1.293 (% body fat)-24.817 in women; leptin = 0.402 (% body fat)-3.087 in men]. Hyperleptinemia was associated with insulin resistance (r = -0.57; P < 0.0001) and high waist to hip ratio (r = 0.75; P < 0.0001) only in men. On the other hand, during the hyperinsulinemic euglycemic clamp studies, hyperinsulinemia acutely increased leptin concentrations (20%) only in women. There was no correlation noted between fasting leptin levels and either resting energy expenditure or insulin-induced thermogenesis in men or women (P = NS). In stepwise and multiple regression models with leptin as the dependent variable, noninsulin-dependent diabetes mellitus did not enter the equations at a statistically significant level. The data indicate that there are important gender-based differences in the regulation and action of leptin in humans. Serum leptin levels increase with progressive obesity in both men and women. However, for any given measure of obesity, leptin levels are higher in women than in men, consistent with a state of relative leptin resistance. These findings have important implications regarding differences in body composition in men and women. The observation that serum leptin is not related to energy expenditure rates suggests that leptin regulates body fat predominantly by altering eating behavior rather than calorigenesis.

The leptin receptor activates janus kinase 2 and signals for proliferation in a factor-dependent cell line

The antiobesity effects of leptin are mediated by the obese receptor (OB-R), a member of the cytokine receptor superfamily. Several isoforms of OB-R that differ in the length of the cytoplasmic domain have been described. An isoform with a long cytoplasmic domain of 302 amino acids, termed OB-Rb, contains the conserved box 1 and box 2 motifs and is likely to be responsible for leptin-induced signaling. A point mutation in the OB-R gene of diabetes (db) mice generates a new splice donor that interferes with the correct splicing of the OB-Rb mRNA and is predicted to cause absence of the OB-Rb protein in db/db mice. Here we examined the signaling potential of the long isoform, OB-Rb, and of a short isoform, OB-Ra, in BaF3 cells, a factor-dependent hematopoietic cell line. The long isoform was able to generate a proliferative signal and upon leptin binding, activated janus kinase 2 (Jak2). Consistently, antibodies directed against the extracellular domain of OB-R coprecipitated Jak2. The short isoform, OB-Ra, was inactive in both proliferation and Jak activation. These results provide further support for the long isoform, OB-Rb, being the principal mediator of the effects of leptin and help to explain why db/db mice are resistant to leptin, despite the presence of the short OB-R isoforms.

Uptake of long chain free fatty acids is selectively up-regulated in adipocytes of Zucker rats with genetic obesity and non-insulin-dependent diabetes mellitus

To examine whether fatty acid transport is abnormal in obesity, the kinetics of [3H]oleate uptake by hepatocytes, cardiac myocytes, and adipocytes from adult male Wistar (+/+), Zucker lean (fa/+) and fatty (fa/fa), and Zucker diabetic fatty (ZDF) rats were studied. A tissue-specific increase in oleate uptake was found in fa/fa and ZDF adipocytes, in which the Vmax was increased 9-fold (p < 0.005) and 13-fold (p < 0.001), respectively. This increase greatly exceeded the 2-fold increase in the surface area of adipocytes from obese animals, and did not result from trans-stimulation secondary to increased lipolysis. Adipocyte tumor necrosis factor-alpha mRNA levels, assayed by Northern hybridization, increased in the order +/+ < fa/fa < ZDF. Oleate uptake was also studied in adipocytes from 20-24-day-old male +/+, fa/+, and fa/fa weanlings. These animals were not obese, and had equivalent plasma fatty acid and glucose levels. Tumor necrosis factor-alpha mRNA levels in +/+ and fa/fa cells also were similar. Nevertheless, Vmax was increased 2.9-fold (p < 0.005) in fa/fa compared +/+ cells. These studies indicate 1) that regulation of fatty acid uptake is tissue-specific and 2) that up-regulation of adipocyte fatty acid uptake is an early event in Zucker fa/fa rats. These findings are independent of the role of any particular fatty acid transporter. Adipocyte mRNA levels of three putative transporters, mitochondrial aspartate aminotransferase, fatty acid translocase, and fatty acid transporting protein (FATP) were also determined; mitochondrial aspartate aminotransferase and FATP mRNAs correlated strongly with fatty acid uptake.

The arcuate nucleus as a primary site of satiety effect of leptin in rats

The obese (ob) gene encodes a fat cell-derived circulating satiety factor (leptin) that is involved in the regulation of energy homeostasis. In the present study, we examined effects of i.c.v. injection of recombinant human leptin on food intake and body weight gain in rats. We also studied effects of direct microinjections of leptin into the arcuate nucleus (Arc), ventromedial hypothalamus (VMH), and lateral hypothalamus (LH). A single i.c.v. injection of recombinant human leptin (0.25-2.0 micrograms/rat) reduced significantly and dose-dependently food intake and body weight gain in rats. Microinjections (0.125-0.5 microgram/site) into the bilateral Arc, VMH, and LH caused dose-related decreases in food intake and body weight gain as compared with vehicle-treated groups with a rank order of potency; Arc > VMH = LH. The present study provides the first direct evidence that the Arc is a primary site of satiety effect of leptin.

Localization of leptin receptor mRNA splice variants in murine peripheral tissues by RT-PCR and in situ hybridization

Expression of leptin receptor splice variants, including the long form variant (Ob-Rb), has been examined in murine peripheral tissues. RT-PCR indicates that the leptin receptor, Ob-R, and in particular the Ob-Ra splice variant are expressed in a wide range of tissues. Expression of the Ob-R receptor was localized by in situ hybridization to specific sites in the spleen, testes, kidney, liver, lung, and adrenal. However, the long form leptin receptor, Ob-Rb, was only expressed at significant levels in the medulla of the adrenal and the inner zone of the medulla of the kidney. The specific sites of expression of different splice variants within peripheral tissues has important implications with regard to the function of leptin.

A short isoform of the human growth hormone receptor functions as a dominant negative inhibitor of the full-length receptor and generates large amounts of binding protein

The GH receptor (GHR) is a member of the cytokine receptor family. Short isoforms resulting from alternative splicing have been reported for a number of proteins in this family. RT-PCR experiments, in human liver and cultured IM-9 cells, using primers in exon 7 and 10 of the GHR, revealed three bands reflecting alternative splicing of GHR mRNA: the predicted product at 453 bp and two other products at 427 and 383 bp. The 427-bp product (GHR1-279) utilized an alternative 3'-acceptor splice site 26 bp downstream in exon 9; the predicted C-terminal residues are six frameshifted exon 9 codons ending in an inframe stop codon. The 383-bp product (GHR1-277) resulted from skipping of exon 9; the predicted C-terminal residues are three frame-shifted exon 10 codons ending in an in-frame stop codon. RNase protection experiments confirmed the presence of the GHR1-279 variant in IM-9 cells and human liver. The proportion of alternative splice to full length was 1-10% for GHR1-279 and less than 1% for GHR1-277. The function of GHR1-279 was examined after subcloning in an expression vector and transient transfection in 293 cells. Scatchard analysis of competition curves for [125l]-hGH bound to cells transfected either with GHR full length (GHRfl) or GHR1-279 revealed a 2-fold reduced affinity and 6-fold increased number of binding sites for GHR1-279. The increased expression of GHR1-279 was confirmed by cross-linking studies. The media of cells transfected with GHR1-279 contained 20-fold more GH-binding protein (GHBP) than that found in the media of cells transfected with the full-length receptor. Immunoprecipitation and Western blotting experiments, using a combination of antibodies directed against extracellular and intracellular GHR epitopes, demonstrated that GHRfl and GHR1-279 can form heterodimers and that the two forms also generate a 60-kDa GHBP similar in size to the GHBP in human serum. Functional tests using a reporter gene, containing Stat5-binding elements, confirmed that while the variant form was inactive by itself, it could inhibit the function of the full-length receptor. We have demonstrated the presence of a splice variant of the GHR in human liver encoding a short form of the receptor similar in size to a protein previously identified in human liver and choroid plexus. Expression studies in 293 cells support the hypothesis that while the expression of the splice variant accounts for only a small proportion of the total GHR transcript, it produces a short isoform that modulates the function of the full-length receptor, inhibits signaling, and generates large amounts of GHBP. The differential expression of GHR receptor short forms may regulate the production of GHBP, and truncated receptors may act as transport proteins or negative regulators of GHR signaling.

Leptin receptor mRNA is expressed in ewe anterior pituitary and adipose tissues and is differentially expressed in hypothalamic regions of well-fed and feed-restricted ewes

Infertility associated with suboptimal nutrition is a major concern among livestock producers. Recently, much effort has been put into understanding the role of the protein leptin in regulating feed intake and reproduction. Leptin, produced by adipocytes, has receptors in the hypothalamus, but more precise locations of leptin receptor-expressing cell bodies have not been reported in a livestock species. The leptin receptor transcript has several splice variants in the mouse and human, but only the "long-form" product (OBRL) is capable of signal transduction. A partial ovine long-form leptin receptor cDNA was cloned and used to evaluate OBRL mRNA expression within hypothalamic, anterior pituitary, and adipose tissues of ovariectomized adult ewes. Expression was detected in reverse transcription-polymerase chain reaction products of all tissues examined. OBRL mRNA was detected by in situ hybridization in the ventromedial and arcuate nuclei of the hypothalamus. In ewes that had been feed restricted for 3 wk before tissue collection, the expression of OBRL mRNA in these areas was greater (P < 0.05) than that found in well-fed ewes. These findings provide evidence that the full-length leptin receptor is expressed in hypothalamic, anterior pituitary, and adipose tissue (the latter proffering an autoregulatory mechanisms for leptin) and that within the hypothalamus, this receptor form is differentially expressed in well-fed vs. feed-restricted animals.

Leptin Stimulates Fetal and Adult Erythroid and Myeloid Development

The ob gene product, leptin, has been shown in several studies to be involved in weight control and recombinant leptin recently has entered clinical trials to treat obesity. The leptin receptor (OB-R/B219) is expressed in a variety of protein isoforms not only in the central nervous system, but also in reproductive, and hematopoietic tissues. We reported recently that the OB-R/B219 was associated with a variety of hematopoietic lineages as well as the small fraction of cells containing the long-term reconstituting hematopoietic stem cells. Herein we report that leptin significantly stimulates the proliferation and differentiation of yolk sac cells and fetal liver cells and stimulates directly hematopoietic precursors. Leptin alone can increase the number of macrophage and granulocyte colonies, and leptin plus erythropoietin act synergistically to increase erythroid development. These data show that leptin has a significant, direct effect on early hematopoietic development and can stimulate the differentiation of lineage-restricted precursors of the erythrocytic and myelopoietic lineages. These observations along with a recent report strongly support our previous hypothesis that leptin has an unanticipated important role in hematopoietic and immune system development.

The rat corpulent (cp) mutation maps to the same interval on (Pgm1-Glut1) rat chromosome 5 as the fatty (fa) mutation

The autosomal recessive obesity mutations fatty (fa) and corpulent (cp) arose in separate rat strains, 13M and Koletsky, respectively. By complementation analysis, the two mutations appear to be in the same gene. The somewhat different phenotypes of fa/fa and cp/cp animals probably reflect the fact that the mutations are segregating on different rat strains. The fa mutation has been mapped to the interval between Pgm1 and Glut1 on rat Chr 5, but cp has not been mapped genetically. We mapped cp in 30 obese progeny of a LA/N-BN cp/+ intercross using microsatellite markers for these flanking genes. Cp maps to the same genetic interval as rat fa and mouse db. Cp is flanked by Glut1 and Pgm1: Pgm1-------- cp -------- Glut1 map distance (cM) 1.67 6.67 Thus, cp and fa map to the same approximately 8 cm interval of the rat genome. In conjunction with the complementation studies alluded to above, these findings indicate that cp and fa are mutations in the same gene (Lepr).

Leptin Stimulates Fetal and Adult Erythroid and Myeloid Development

The ob gene product, leptin, has been shown in several studies to be involved in weight control and recombinant leptin recently has entered clinical trials to treat obesity. The leptin receptor (OB-R/B219) is expressed in a variety of protein isoforms not only in the central nervous system, but also in reproductive, and hematopoietic tissues. We reported recently that the OB-R/B219 was associated with a variety of hematopoietic lineages as well as the small fraction of cells containing the long-term reconstituting hematopoietic stem cells. Herein we report that leptin significantly stimulates the proliferation and differentiation of yolk sac cells and fetal liver cells and stimulates directly hematopoietic precursors. Leptin alone can increase the number of macrophage and granulocyte colonies, and leptin plus erythropoietin act synergistically to increase erythroid development. These data show that leptin has a significant, direct effect on early hematopoietic development and can stimulate the differentiation of lineage-restricted precursors of the erythrocytic and myelopoietic lineages. These observations along with a recent report strongly support our previous hypothesis that leptin has an unanticipated important role in hematopoietic and immune system development.

Suggestive linkages between markers on human 1p32-p22 and body fat and insulin levels in the Quebec Family Study

A single-gene rodent mutation (diabetes) and a quantitative trait locus (dietary obese 1) mapped to the mid portion of mouse chromosome 4 have been related to obesity and/or insulin levels. Synteny relationships place their putative human homologs on 1p31 and 1p35-p31, respectively. In 137 sibships of adult brothers and sisters from the Québec Family Study, genetic linkages between seven microsatellite markers from 1p32-p22 and various obesity- and diabetes-related quantitative phenotypes were examined using single locus sibpair linkage analysis. Suggestive linkages were observed between markers D1S476 and body mass index (p = 0.05), fat mass (p = 0.02), the sum of six skinfolds (p = 0.02), the insulin area after an oral glucose tolerance test (p = 0.02), and between the neighboring marker D1S200 and body mass index (p = 0.03), and fat mass (p = 0.009). Suggestive linkages were also observed between the more telomeric markers D1S193 and body mass index (p = 0.03), and between the neighboring marker D1S197 and fasting insulin level (p = 0.05). No linkage was observed with the trunk to extremity skinfolds ratio. These linkages suggest that human homologs of the mouse diabetes or dietary obese 1 and/or other genes in this interval on chromosome 1 play a role in the regulation of body mass, body composition, and insulin levels, but not of subcutaneous fat distribution.

Augmentation of obese (ob) gene expression and leptin secretion in obese spontaneously hypertensive rats (obese SHR or Koletsky rats)

To explore the pathophysiologic roles of the obese (ob) gene product, leptin, in the development of obesity and hypertension, we examined ob gene expression and leptin secretion in obese spontaneously hypertensive rats (obese SHR or Koletsky rats) at the stage of established obesity and hypertension. Expression of the ob gene was augmented in the epididymal, mesenteric, subcutaneous, and retroperitoneal white adipose tissue (WAT) from 20-week-old male obese SHR compared to their lean littermates (lean SHR). Using a radioimmunoassay for rat leptin, we also measured plasma leptin levels in 20-week-old lean and obese SHR. Plasma leptin levels in obese SHR (292.5 +/- 37.1 ng/ml) were more than 100-fold higher than those in lean SHR (2.8 +/- 1.0 ng/ml). The present study demonstrates that ob gene expression and leptin secretion are markedly augmented in obese SHR.

Expression of leptin and beta 3-adrenergic receptors in rat adipose tissue in altered thyroid states

The level of leptin [the obese (ob) gene product] mRNA is markedly elevated in hypothyroid male rats. The administration of tri-iodothyronine (T3) to hypothyroid rats resulted in a 40% decrease in leptin mRNA at 8 h. This decrease in leptin mRNA was associated with a parallel decline in circulating leptin levels of about 50% at 24 h. Conversely, beta 3-adrenergic receptor mRNA levels were markedly decreased in epididymal adipose tissue from hypothyroid rats. T3 administration resulted in a 147% increase at 12 h in beta 3-adrenergic receptor mRNA. There was a corresponding increase due to T3 in the lipolytic response to the specific beta 3-adrenergic agonist CL 316,243 that paralleled the increase in beta 3-adrenergic receptor mRNA. T3-mediated changes in leptin and beta 3-adrenergic receptor mRNAs were blocked by cycloheximide, suggesting the involvement of short-lived proteins in these effects. The present results indicate that T3 has opposite effects to those of insulin on the white adipose tissue of rats with respect to leptin mRNA expression.

Leptin receptor (OB-R) signaling. Cytoplasmic domain mutational analysis and evidence for receptor homo-oligomerization

The leptin receptor (OB-R) mediates the weight regulatory effects of the adipocyte secreted hormone leptin (OB). Previously we have shown that the long form of OB-R, expressed predominantly in the hypothalamus, can mediate ligand-induced activation of signal transducer and activator of transcription factors 1, 3, and 5 and stimulate transcription via interleukin-6 and hematopoietin receptor responsive gene elements. Here we report that deletion and tyrosine substitution mutagenesis of OB-R identifies two distinct regions of the intracellular domain important for signaling. In addition, granulocyte-colony stimulatory factor receptor/OB-R and OB-R/granulocyte-colony stimulatory factor receptor chimeras are signaling competent and provide evidence that aggregation of two OB-R intracellular domains is sufficient for ligand-induced receptor activation. However, signaling by full-length OB-R appears to be relatively resistant to dominant negative repression by signaling-incompetent OB-R, suggesting that mechanisms exist to permit signaling by the long form of OB-R even in the presence [corrected] of excess naturally occurring short form of OB-R.

A short form of leptin receptor performs signal transduction

The obese (ob) gene product, leptin, a peptide hormone, which is synthesized in adipocytes, is a satiety factor and is involved in the control of body weight via the regulation of energy homeostasis. Several alternate spliced isoforms (a-e, as well as others) of the leptin receptor (OBR) have been cloned, all of which, except for OBRe (soluble form), contain a single transmembrane domain. They share the same extracellular domain, with homology to the class I cytokine receptor family. The OBRb, which has longest cytoplasmic domain, is expressed in high levels in the hypothalamus and is thought to be the only isoform capable of signal transmission. Herein, we report the mRNA expression of immediate early genes, c-fos, c-jun and jun-B, which are induced by leptin addition, not only in CHO cells expressing the OBRb, but also in cells expressing one of the short form receptors, OBRa.

Relatively low plasma leptin concentrations precede weight gain in Pima Indians

Leptin, the product of the ob gene, is a hormone, produced by adipose cells, that inhibits food intake and increases energy expenditure in rodents. In humans, plasma leptin concentrations correlate closely with the size of the adipose tissue depot; however, there is considerable variation in plasma leptin concentrations at any given degree of fatness. To investigate whether individuals prone to weight gain are hypoleptinemic, we measured fasting plasma leptin concentrations in two groups of weight-matched nondiabetic Pima Indians followed for approximately 3 years, 19 of whom subsequently gained weight and 17 of whom maintained their weight. After we adjusted for initial percent body fat, mean plasma leptin concentration was lower in those who gained weight than in those whose weight was stable. These data indicate that relatively low plasma leptin concentrations may play a role in the development of obesity in Pima Indians, a population prone to obesity.

The signal transducer gp130 is shared by interleukin-6 family of haematopoietic and neurotrophic cytokines

Receptors for many of the cytokines functioning in the haematopoietic system belong to the class I cytokine receptor family. In most cases these receptors share common signal transducing receptor components in the same family, which explains the functional redundancy of haematopoietic cytokines. Interleukin-6 and related cytokines, interleukin-11, leukaemia inhibitory factor, oncostatin M, ciliary neurotrophic factor and cardiotrophin-1, are all pleiotrophic, from the haematopoietic to the nervous system, and exhibit overlapping biological activities. Receptors for these cytokines fall into the class I cytokine receptor family. Functional receptor complexes for the interleukin-6 family of cytokines share a membrane glycoprotein 130 (gp130) as a critical component for signal transduction. In these receptor complexes, gp130 and ligand-specific chains possess no intrinsic tyrosine kinase domain but are associated with cytoplasmic tyrosine kinases. Ligand stimulation triggers homo- or heterodimerization of gp130, leading to activation of the associated cytoplasmic tyrosine kinases and subsequent modification of transcription factors. This paper reviews the recent progress in the study of gp130 and the background information from biomedical and biochemical viewpoints.

Non-insulin-dependent diabetes mellitus--a collision between thrifty genes and an affluent society

Non-insulin-dependent diabetes mellitus (NIDDM) is one of the most common non-communicable diseases in the world. It has become obvious that NIDDM is the result of a collision between thrifty genes and an affluent society. Genes predisposing to NIDDM might have been survival genes for our ancestors, helping them to store energy during long periods of starvation. When these genes are exposed to a sedentary lifestyle and high caloric intake typical to the Western world, they predispose to obesity and insulin resistance. NIDDM results when beta cells cannot compensate for insulin resistance by increasing insulin secretion. Therefore, at least two inherited defects can be expected in NIDDM, one causing obesity and insulin resistance and the other inability to increase insulin secretion. In reality there may be more inherited defects. It has become quite clear that diabetes cannot simply be divided into NIDDM and insulin-dependent diabetes mellitus (IDDM). The disease is more heterogeneous; unmasking this heterogeneity and identifying new subgroups of diabetes presents a challenge to modern molecular biology.

Structure and function in gene patenting

The United States patent system treats DNA sequences as large chemical compounds in determining their patentability. This approach has been helpful to those who seek to patent previously unidentified DNA sequences, but it may prove less advantageous from the perspective of those who elucidate biological functions and disease relevance of previously identified genes. A current controversy over patent rights for DNA sequences encoding leptin receptors provides a useful case study for illustrating some of the issues that are likely to arise in applying doctrine derived from chemical patent cases in the context of gene discovery.

Developmental and nutritional changes of ob and PPAR gamma 2 gene expression in rat white adipose tissue

The ob gene encodes leptin, a hormone which induces satiety and increases energy expenditure. The peroxisome proliferator-activated receptor gamma 2 isoform (PPAR gamma 2) gene encodes a transcription factor which controls adipocyte differentiation and expression of fat-specific genes. We have studied the regulation of these two genes in white adipose tissue (WAT) during the suckling-weaning transition. Suckling rats ingest a high-fat diet (milk). Fat-pad weight barely varied during the last week of suckling. ob mRNA levels, which were very low in 15-day-old rats, rose approximately 6-fold until weaning at 21 days. When the rats were weaned on to a standard (high-carbohydrate) laboratory chow, epididymal WAT enlarged approximately 7-fold, and ob mRNA kept increasing progressively and doubled between 21 and 30 days. This evolution contrasted with that of fatty acid synthase (FAS) mRNA, which increased sharply, but only after weaning. To distinguish between the influence of developmental and nutritional factors on ob expression, a group of rats was weaned on to a high-fat diet. This prevented the rise in glycaemia and insulinaemia and the decrease in plasma non-esterified fatty acids which otherwise occurred at weaning. This also resulted in a slight (10-15%) decrease in food intake and body weight gain. Under this high-fat diet, the rise of ob mRNA in WAT was augmented (3.7-fold in 30- versus 21-day-old pups), whereas the normal rise in FAS mRNA levels was attenuated. Fat-pad weights and adipocyte cell size and number were roughly similar in high-carbohydrate- and high-fat-weaned pups. mRNA levels of PPAR gamma 2, like those of ob, were low in the WAT of 15-day-old suckling pups, doubled at 21 days, and reached a maximum as soon as 23 days. This evolution further differed from that of ob mRNA in not being influenced by diet composition. In conclusion, ob expression markedly increases during the suckling-weaning transition, and this effect is accentuated by a high-fat diet. Qualitative nutritional changes in ob mRNA were correlated with neither acute changes in adipose-tissue mass, nor cell size/number, nor variations in insulinaemia. PPAR gamma 2 also increased during suckling, but rapidly reached a plateau after weaning and no longer changed thereafter. Unlike ob, PPAR gamma 2 was not influenced by the diet composition.

ob gene expression and secretion of leptin following differentiation of rat preadipocytes to adipocytes in primary culture

Expression of the ob gene and production of leptin have been examined on differentiation of rat fibroblastic preadipocytes to adipocytes in primary culture. Preadipocytes were obtained from the inguinal fat pad of suckling rats, and following differentiation the cells contained lipid droplets and the mRNAs for both lipoprotein lipase and adipsin were detected by Northern blotting. ob mRNA was not, however, detected on Northern blots, but analysis by RT-PCR indicated that the ob gene was expressed, particularly after differentiation. Measurement of leptin in the culture medium by ELISA showed that the ob gene product was secreted by adipocytes from approximately 4 days after the induction of differentiation. Leptin production was sustained over a 2-week period with a peak at 8-10 days post-induction. Dexamethasone stimulated leptin production, while an inhibition was observed with the beta-adrenoceptor agonist isoprenaline. These results demonstrate that following the differentiation of fibroblastic preadipocytes to adipocytes in primary culture, leptin is secreted with the cells responding to stimuli which regulate production of the hormone.

Targeted disruption of the melanocortin-4 receptor results in obesity in mice

The melanocortin-4 receptor (MC4-R) is a G protein-coupled, seven-transmembrane receptor expressed in the brain. Inactivation of this receptor by gene targeting results in mice that develop a maturity onset obesity syndrome associated with hyperphagia, hyperinsulinemia, and hyperglycemia. This syndrome recapitulates several of the characteristic features of the agouti obesity syndrome, which results from ectopic expression of agouti protein, a pigmentation factor normally expressed in the skin. Our data identify a novel signaling pathway in the mouse for body weight regulation and support a model in which the primary mechanism by which agouti induces obesity is chronic antagonism of the MC4-R.

Rab 3D in rat adipose cells and its overexpression in genetic obesity (Zucker fatty rat)

Members of the Rab 3 subfamily of low-molecular-mass GTP-binding proteins have been functionally implicated in regulated exocytosis. The aim of the present study was to examine the subcellular distribution of a member of this family, Rab 3D, in rat adipose cells, given the hypothesis that this protein might be involved in insulin-stimulated GLUT4 exocytosis. We show that Rab 3D immunoreactivity is associated predominantly with the high-density microsomal fraction, where the signal intensity is 3- and 7-fold greater than that in plasma membranes and low-density microsomes respectively. Rab 3D does not co-localize with GLUT4 on immuno-isolated intracellular vesicles and, unlike GLUT4, it is not redistributed in response to insulin. Thus, if Rab 3D plays a role in GLUT4 trafficking, it relies on mechanisms independent of relocation. We observed that Rab 3D is overexpressed in adipose cells of obese (fa/fa) Zucker rats, in a tissue- and isoform-specific manner. The pathophysiological significance of this defect remains elusive. This could form the molecular basis for altered adipose secretory function in obesity.

Human blood-brain barrier leptin receptor. Binding and endocytosis in isolated human brain microvessels

The peripheral production of leptin by adipose tissue and its putative effect as a signal of satiety in the central nervous system suggest that leptin gains access to the regions of the brain regulating energy balance by crossing the brain capillary endothelium, which constitutes the blood-brain barrier in vivo. The present experiments characterize the binding and internalization of mouse recombinant leptin in isolated human brain capillaries, an in vitro model of the human blood-brain barrier. Incubation of 125I-leptin with isolated human brain capillaries resulted in temperature-dependent binding: at 37 degrees C, approximately 65% of radiolabeled leptin was bound per milligram of capillary protein. Two-thirds of the bound radioactivity was resistant to removal by acid wash, demonstrating endocytosis of 125I-leptin into capillary cells. At 4 degrees C, binding to isolated capillaries was reduced to approximately 23%/mg of protein, the majority of which was acid wash resistant. Binding of 125I-leptin to brain capillary endothelial plasma membranes was saturable, described by a two-site binding model with a high-affinity dissociation constant of 5.1+/-2.8 nM and maximal binding capacity of 0.34+/-0.16 pmol/mg of membrane protein. Addition of porcine insulin or insulin-like growth factor at a final concentration of 100 nM had a negligible effect on leptin binding. These results provide evidence for a leptin receptor that mediates saturable, specific, temperature-dependent binding and endocytosis of leptin at the human blood-brain barrier.

Brown adipose tissue, beta 3-adrenergic receptors, and obesity

Brown adipose tissue is distinguished by its unique capacity for uncoupled mitochondrial respiration, which is highly regulated by sympathetic nerve activity. Because of this, energy expenditure in brown fat is capable of ranging over many orders of magnitude. The fact that the function of brown adipose tissue is impaired in obese rodents and that transgenic mice with decreased brown fat develop obesity demonstrates the importance of brown fat in maintaining nutritional homeostasis. However, the role of brown fat in humans is less clear. beta 3-Adrenergic receptors are found on brown adipocytes, and treatment with beta 3-selective agonists markedly increases energy expenditure and decreases obesity in rodents. Whether beta 3-selective agonists will be effective anti-obesity agents in humans is presently under investigation.

Obese (ob) gene defects are rare in human obesity

Our knowledge of the role of the recently cloned ob-protein (leptin) in the regulation of body fat stores is largely derived from experiments performed in mice. Different mouse models exhibit abnormalities in ob-gene expression, with extreme overexpression in mice which lack bioactive ob-protein, have nonfunctional ob-receptors or hypothalamic lesions, and undetectable expression in mice with suggested defects in regulatory elements. The aim of this study is to examine if defects, corresponding to those in mice, exist in human obesity. Adipose tissue was obtained from 94 adult obese subjects and from six children who had developed obesity after surgery in the hypothalamic region. Total RNA was isolated and ob-gene expression was examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and Northern blot. The coding region of the ob-gene was sequenced in both directions in the 94 obese adults. No mutations were detected in the coding region of the ob-gene and ob-gene expression was detectable in all subjects and none of the subjects had an extreme overexpression. There was no systematic increase in ob-expression in obese children with hypothalamic disease compared to their healthy brothers and sisters. These results show that severe abnormalities involving the ob-gene, analogous to those described in mouse models, are rare in human obesity. We therefore conclude that the cloning and subsequent analysis of the ob-gene has not provided information that can, by itself, explain the genetic component in the development of human obesity.

Gp130 and the interleukin-6 family of cytokines

Receptors for most interleukins and cytokines that regulate immune and hematopoietic systems belong to the class I cytokine receptor family. These molecules form multichain receptor complexes in order to exhibit high-affinity binding to, and mediate biological functions of, their respective cytokines. In most cases, these functional receptor complexes share common signal transducing receptor components that are also in the class I cytokine receptor family, i.e. gp130, common beta, and common gamma molecules. Interleukin-6 and related cytokines, interleukin-11, leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and cardiotrophin-1 are all pleiotropic and exhibit overlapping biological functions. Functional receptor complexes for this interleukin-6 family of cytokines share gp130 as a component critical for signal transduction. Unlike cytokines sharing common beta and common gamma chains that mainly function in hematopoietic and lymphoid cell systems, the interleukin-6 family of cytokines function extensively outside these systems as well, e.g. from the cardiovascular to the nervous system, owing to ubiquitously expressed gp130. Stimulation of cells with the interleukin-6 family of cytokines triggers homo- or hetero-dimerization of gp130. Although gp130 and its dimer partners possess no intrinsic tyrosine kinase domain, the dimerization of gp130 leads to activation of associated cytoplasmic tyrosine kinases and subsequent modification of transcription factors. This paper reviews recent progress in the study of the interleukin-6 family of cytokines and gp130.

Beta-3 adrenoceptor (beta-3AR) expression in leptin treated OB/OB mice

Leptin-deficient Ob/Ob mice are hypometabolic and have reduced fat cell expression of beta-3 adrenoceptors (ARs). To determine whether leptin repletion restores beta-3 AR number, C57BL/6J Ob/Ob mice were given exogenous leptin (5 mg/kg I.P. daily) for 21 days. Leptin administration reduced body weight from 43.1+/-3.7 to 34.1+/-3.7 g in Ob/Ob animals but had no effect on weight in wildtype animals. Body weight increased by 12% in Ob/Ob mice receiving saline. Beta-3 AR mRNA concentrations were markedly reduced in Ob/Ob animals at baseline. Leptin increased beta-3 AR mRNA to control levels in Ob/Ob mice, but had no effect in wildtype animals. Adipocyte leptin mRNA was increased by 400% in Ob/Ob mice and did not suppress with exogenous leptin administration, suggesting no direct feedback regulation of leptin synthesis. We speculate that restoration of beta-3 AR expression by repleting leptin may be important in correcting hypometabolism in Ob/Ob animals.

Leptin can induce proliferation, differentiation, and functional activation of hemopoietic cells

Many cytokines exert their biological effect through members of the hemopoietin receptor family. Using degenerate oligonucleotides to the common WSXWS motif, we have cloned from human hemopoietic cell cDNA libraries various forms of the receptor that was recently shown to bind the obesity hormone, leptin. mRNAs encoding long and short forms of the human leptin receptor were found to be coexpressed in a range of human and murine hemopoietic organs, and a subset of cells from these tissues bound leptin at the cell surface. Ectopic expression in murine Ba/F3 and M1 cell lines revealed that the long, but not the short, form of the leptin receptor can signal proliferation and differentiation, respectively. In cultures of murine or human marrow cells, human leptin exhibited no capacity to stimulate cell survival or proliferation, but it enhanced cytokine production and phagocytosis of Leishmania parasites by murine peritoneal macrophages. Our data provide evidence that, in addition to its role in fat regulation, leptin may also be able to regulate aspects of hemopoiesis and macrophage function.

Attenuation of the obesity syndrome of ob/ob mice by the loss of neuropeptide Y

The obesity syndrome of ob/ob mice results from lack of leptin, a hormone released by fat cells that acts in the brain to suppress feeding and stimulate metabolism. Neuropeptide Y (NPY) is a neuromodulator implicated in the control of energy balance and is overproduced in the hypothalamus of ob/ob mice. To determine the role of NPY in the response to leptin deficiency, ob/ob mice deficient for NPY were generated. In the absence of NPY, ob/ob mice are less obese because of reduced food intake and increased energy expenditure, and are less severely affected by diabetes, sterility, and somatotropic defects. These results suggest that NPY is a central effector of leptin deficiency.

Exercise as treatment for obesity

In recent times, affluent societies have become less physically active, and this has undoubtedly contributed to the increased incidence of obesity. Formal programs of exercise training can reduce body weight and fat, but, in many cases, the changes produced by exercise are small. When combined with energy restriction, exercise results in little further weight loss, but there is a strong trend for a greater loss of body fat. Thus, during diet-induced weight loss, added exercise seems to accelerate fat loss and maintain lean body mass, a condition which may prevent a decline in RMR. It is becoming increasingly clear that weight loss is better maintained when exercise is part of a weight-reducing program. Furthermore, following a period of diet-induced weight loss, participation in regular exercise amounting to an energy expenditure of more than 1500 kcal/week will result in more successful maintenance of the lesser weight. An emphasis should be placed on adopting life-long habits conducive to weight control and overall health rather than temporary measures for weight loss. A program which encompasses regular physical activity, modest energy intake, and reduced calories from fat has the potential to meet such a goal. Regular physical activity has the potential to reverse insulin resistance, improve cardiovascular function and the blood lipid profile, and control high blood pressure. Overweight individuals can obtain these important benefits even if body weight is not completely normalized during a program of regular physical activity. This should help alleviate problems of diabetes, heart disease, and hypertension often associated with being overweight. Further research is needed to identify more specifically the optimal amount, type, and intensity of exercise needed to produce weight loss or maintain ideal body weight. To date, the best recommendation comes from the American College of Sports Medicine. Persons are urged to engage in regular physical activity which promotes a daily energy expenditure of at least 300 kcal/day and to choose from a variety of activities, in particular, those which are enjoyable and that can be continued for life.

The endocrinology of obesity

Numerous endocrine alterations are associated with obesity (Table 1). The majority of the alterations are secondary to obesity and must be considered simply associated and potentially in the pathogenesis of the complications of obesity. The discovery of new endocrine peptides such as leptin that signal body fat content will increase our understanding of the regulation of body fat content. As a result, therapies will most certainly be developed that are directly targeted at the alterations in endocrine function.

Potassium channel dysfunction in hypothalamic glucose-receptive neurones of obese Zucker rats

1. We have shown, using intracellular and cell-attached recordings, that glucose-receptive (GR) neurones of obese Zucker rats exhibit abnormal electrophysiological responses to changes in extracellular glucose concentration, whereas GR neurones of lean Zucker and control rats respond normally. 2. In inside-out recordings from obese rat GR neurones it was shown that the 150 pS ATP-sensitive K+ (KATP) and the 160 pS calcium-activated K+ (KCa) channels were absent, whereas both were present in GR neurones of lean Zucker and control rats. 3. The potassium channel most frequently observed in inside-out patches from obese GR neurones was characterized by a conductance of 213 pS, was activated by raising internal calcium and inhibited by application of internal ATP. This channel (which we have termed Kfa) was not observed in lean or control rat GR neurones. 4. Tolbutamide (100 microM) was found to induce no effect or to elicit a small depolarization of obese rat GR neurones in the absence of glucose, in contrast to its clear excitatory actions on control or lean Zucker GR neurones. 5. Intracellular, cell-attached and inside-out recordings from obese rat non-GR neurones showed that there was no alteration in their membrane properties or firing characteristics or in the characteristics of the large-conductance calcium-activated K+ channel (KCa) present in these neurones as compared with lean and control rats. 6. It is concluded that the Kfa channel is specific to GR neurones of obese Zucker rats and that the presence of this channel coupled with the absence of KATP and KCa channels results in the abnormal glucose-sensing response of these neurones.

Leptin and OB-R: body weight regulation by a cytokine receptor

There has been intense recent interest in the molecules and pathways governing mammalian body weight regulation. Leptin (OB), an ancestral member of the cytokine family, is an adipocyte-secreted circulating hormone exhibiting weight regulatory properties. Recently, the leptin receptor (OB-R) was identified and shown to exhibit sequence homology and functional similarity to members of the class I cytokine receptor family. The mechanisms governing OB-R triggering and signal transduction have begun to be elucidated, providing new insight into the pathways controlling mammalian body weight homeostasis.

Genomic structure of the human OB receptor and identification of two novel intronic microsatellites

Identification of the OB (leptin) receptor (OBR) as the gene that is defective in diabetes (Leprdb) mice and fatty (Leprfa) rats provides an important candidate gene for the study of the genetics of human obesity. We defined the boundaries of the 18 coding exons for the long form of OBR, and sequenced the immediately adjacent intronic regions. These sequences can be used to generate reagents for genetic analysis (e.g., direct sequencing, single-stranded conformational polymorphism analysis, etc.) of the possible role of OBR in the regulation of adiposity in humans. In addition, we have identified two highly polymorphic intronic microsatellites that can be scored with the polymerase chain reaction.

Lessons from animal models of obesity

Obesity in animals may result from genetic, dietary, or neuroendocrine perturbations. Study of these models has identified the central systems that regulate food intake and energy expenditure and identified the interdependence of feeding behavior, the autonomic nervous system, and adrenal glucocorticoids in the development of obesity. The animal models of obesity have been influential in showing that adipose tissue is an important secretory tissue. The recent identification of five genes that cause obesity will provide new insight into the physiologic systems that regulate energy balance.

Modulation of insulin activities by leptin

Leptin mediates its effects on food intake through the hypothalamic form of its receptor OB-R. Variants of OB-R are found in other tissues, but their function is unknown. Here, an OB-R variant was found in human hepatic cells. Exposure of these cells to leptin, at concentrations comparable with those present in obese individuals, caused attenuation of several insulin-induced activities, including tyrosine phosphorylation of the insulin receptor substrate-1 (IRS-1), association of the adapter molecule growth factor receptor-bound protein 2 with IRS-1, and down-regulation of gluconeogenesis. In contrast, leptin increased the activity of IRS-1-associated phosphatidylinositol 3-kinase. These in vitro studies raise the possibility that leptin modulates insulin activities in obese individuals.

Obesity genes and the regulation of body fat content

Physiological investigation has demonstrated that the central nervous system monitors body composition and adjusts energy intake and expenditure to stabilize total adipose tissue mass. Genetic variations in the signalling molecules involved in this regulatory system account for the heritable component of body fat content. The application of molecular techniques to rodent models of Mendelian obesity has resulted in the characterization of five loci at which mutations produce an abnormal accumulation of body fat. The genes at these loci include agouti, which encodes a molecule that antagonizes the binding of alpha melanocyte-stimulating hormone to its receptor; fat, which encodes carboxypeptidase E; tubby, which encodes a putative phosphodiesterase; obese, which encodes a circulating satiety protein; and diabetes, which encodes the receptor for the obese gene product. A more detailed understanding of the functional interrelationships of these genes should lead to important new insights into the causes and potential therapies for human obesity.